Under normal operating conditions, electrical transmission and distribution equipment is subject to voltages within a relatively narrow range. Due to lightning strikes, switching surges or other system disturbances, portions of the electrical network may experience momentary or transient voltage levels that greatly exceed the levels experienced by the equipment during normal operating conditions. Left unprotected, critical and costly equipment such as transformers, switching apparatus, computer equipment, and electrical machinery may be damaged or destroyed by such over-voltages and the resultant current surges. Accordingly, it is routine practice to protect such apparatus from dangerous over-voltages through the use of surge arresters.
A surge arrester is a protective device that is commonly connected in parallel with a comparatively expensive piece of electrical equipment so as to limit overvoltages and shunt or divert the over-voltage induced current surges safely around the equipment, thereby protecting the equipment and its internal circuitry from damage. When caused to operate, a surge arrester forms a current path to ground having very low impedance relative to the impedance of the equipment that it is protecting. In this way, current surges which otherwise would give high overvoltages across the equipment would be diverted through the arrester to ground.
Conventional surge arresters typically include an elongate outer housing made of an electrically insulating material (porcelain or nowadays more commonly polymer), a pair of electrical terminals at opposite ends of the housing for connecting the arrester between a line-potential conductor and ground, and an array of electrical components in the housing that form a series path between the terminals. These components typically include a stack of voltage-dependent, nonlinear resistive elements. These nonlinear resistors or varistors are characterized by having a relatively high resistance at the normal steady-state voltage and a much lower dynamic resistance when the arrester is subjected to transient overvoltages. Depending on the type of arrester, it may also include one or more electrodes, heat sinks or spark gap assemblies housed within the insulated housing and electrically in series with the varistors.
A substantially uniform voltage gradient along the arrester connected to a high tension terminal is obtained by using grading rings or within the arrester housing using a high number of small capacitors which are connected physically and electrically in parallel to the nonlinear resistive elements.